Microstrip and stripline are increasingly popular transmission line media for short distance, low power applications, including RF microwave circuits and high speed digital circuits. Stripline consists of a printed conductor between two ground planes, typically formed from copper-clad polyethylene sheets. Electrically, stripline has properties similar to coaxial cable transmission lines.
Microstrip, a popular transmission line due to ease of fabrication and circuit assembly, uses dielectric substrates with a metal either deposited on or etched away to form the circuit line conductors. Microstrip has a single ground plane with a dielectric layer sandwiched between the circuit conductors and the ground plane. Generally, the attentuation characteristics of microstrip are greater than that of stripline, and also the power handling capability of microstrip is lower. Stripline provides inherently better crosstalk (i.e., neighboring channel interference) performance because the field within a stripline conductor is totally contained between the two ground planes. Although crosstalk interference is greater with microstrip, the exposed microstrip conductors are convenient for attaching circuit elements, and thus microstrip is typically used for applications requiring the attachment of circuit elements to the board. However, to reduce crosstalk in a microstrip embodiment, thin signal traces and a thin microstrip board are used.
In the prior art, microstrip is used to provide RF signals to transducers of an acousto-optic Bragg cell. In some applications, the Bragg cell may have 64, 128, or more, transducers each to receive a different RF signal. Because the microstrip conductors are exposed, it is relatively easy to attach the Bragg cell transducers thereto, but crosstalk performance is less than optimum because the microstrip has a conductive ground plane on only one surface. Microstrip can also be used to interface VHSIC (Very High-Speed Integrated Circuit) elements and to connect VHSIC elements to other devices. Because microstrip is a form of transmission line, it provides the necessary high-speed performance for VHSIC configured devices or any high-speed circuits.
Because stripline offers improved crosstalk performance characteristics over microstrip, it is desirable to use stripline in lieu of microstrip whenever possible. Since the stripline conductor is sandwiched between the two ground planes, access thereto creates difficulties. One technique for resolving these disadvantages is disclosed in U.S. Pat. No. 3,209,291. In this invention the circuit element is built directly into the center conductor of the balanced stripline transmission line by cutting a hole in the center conductor and placing the element within the hole. U.S. Pat. No. 4,085,390 discloses assembling the discrete components on the center conductor and cutting away a pocket in the dielectric to provide clearance for these circuit components. Lastly, U.S. Pat. No. 3,792,383 discloses a hybrid stripline using discrete and distributed circuit components. The discrete components are fixed to one ground plane with the leads protruding through openings in the ground plane and the attached dielectric. The leads are soldered to the circuit between the ground planes by using an access opening in the ground plane on the opposite side of the circuit board from where the discrete component is mounted. This access opening is sufficiently large to receive a soldering tip for soldering the component lead to the stripline circuit.
In another stripline embodiment access holes can be provided to the inner conductor by plated-through holes from one of the ground planes. A disadvantage associated with this technique is that the two boards comprising the stripline board are ideally extremely thin (to reduce crosstalk) and the copper conductor thickness is also reduced, thus the plated-through holes have a very brittle contact with the inner lines. It is therefore easy for this contact to be broken by board flexing associated with normal use.
It is well-known in the art that a standard printed circuit board can accommodate leads spaced at approximately 10 mils but such printed circuit boards do not provide acceptable high-frequency performance. At high frequencies the circuit traces act as distributed circuit elements, creating impedance problems, and the close spacing degrades the crosstalk performance.